1. Field of the Invention
The present invention relates to a transfer mechanism for transferring a target object, such as a semiconductor wafer, and a semiconductor processing system employing the transfer mechanism. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target object, such as a semiconductor wafer or a glass substrate used for an LCD (Liquid Crystal Display) or FPD (Flat Panel Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target object.
2. Description of the Related Art
In the process of manufacturing semiconductor devices, a semiconductor wafer is subjected to various semiconductor processes, such as film formation, etching, oxidation, diffusion, and so forth. In these processes, owing to the demands of increased miniaturization and integration of semiconductor devices, the throughput and yield involving these processes need to be increased. In light of this, there is a semiconductor processing system of the so-called cluster tool type, which has a plurality of process chambers for performing the same process, or a plurality of process chambers for performing different processes, connected to a common transfer chamber. With this system, various steps can be performed in series, without exposing a wafer to air.
Such a processing system has a transfer unit by which a wafer is automatically transferred from one place to another. The transfer unit has an arm with a pick at the distal end, which is, for example, extendable/retractable, swingable, and movable up and down. The pick directly supports a wafer, and moves horizontally to a transfer position, so that the wafer is transferred to a predetermined place.
It is necessary to prevent the pick and a wafer placed thereon from interfering or colliding with other members, while the transfer unit is moving. It is also necessary for the pick to properly pick up a wafer placed at a certain place, transfer it to a destination, and place it at an appropriate position, with high positional accuracy.
For example, a common transfer chamber is provided with optical sensors at the ceiling or flooring in front of process chambers. The optical sensors are used to detect the presence/absence of a target object on a pick, so as to control the operation of gate valves disposed between the common transfer chamber and the respective process chambers.
Jpn. Pat. Appln. KOKAI Publication No. 10-223732 (Patent Document 1) discloses an example of the processing systems of this kind. In this system, an optical sensor is disposed near the entrance of a specific one of the process chambers. The optical sensor is used to detect whether a pick supports a target object with sufficient positional accuracy.
Jpn. Pat. Appln. KOKAI Publication No. 2001-338969 (Patent Document 2) discloses another example of the processing systems of this kind. In this system, a common transfer chamber is provided with a line sensor, while a transfer unit is provided with a wing. The line sensor is used to detect the positional relationship of a target object relative to the wing, so as to detect whether a pick supports the target object with sufficient positional accuracy. If certain positional misalignment is found, the transfer unit is controlled in operation to compensate for the positional misalignment amount.
However, the conventional mechanism for detecting positional misalignment of a target object requires not only an optical sensor for detecting the presence/absence of a target object on a pick, but also an optical sensor for detecting the amount of positional misalignment of the target object. Accordingly, the mechanism for detecting positional misalignment of a target object increases the initial cost of the system. Further, sometimes it may be necessary to dispose an optical sensor or line sensor at a position deviated from the transfer route of a target object. In this case, when detection of positional misalignment of the target object is performed, the target object has to be transferred through the position deviated from the transfer route. This increases the transfer time period of the target object, and thus decreases the throughput.
Furthermore, for example, in the case of a wafer (target object) of 300 mm or 200 mm, the diameter thereof is required to have a high dimensional accuracy of ±0.2 mm. Under the circumstances, a strict design rule tends to be applied to transfer of target objects, such that the transfer positional accuracy is set to be within a range of ±0.2 mm. However, the diameter of target objects may fluctuate within a range of ±0.2 mm, as described above. This fluctuation in the diameter of target objects due to individual difference can affect the positional accuracy in transfer of target objects.